This invention relates to instrumentation for an automotive vehicle, particularly for a truck in which the engine drives the wheels through a manually shifted transmission. The invention is concerned with providing the vehicle operator with a warning of potentially damaging motoring of the engine so that he will have an opportunity to avoid the potentially destructive effects of such motoring.
Internal combustion engines of the types commonly in use today in vehicles such as on-road trucks, off-road trucks, construction equipment, automobiles, etc., are capable of operating over a considerable range of rpm (revolutions per minute). For any given engine, there is an upper limit to this range, and this limit is usually dictated by certain components of the engine. A typical limiting factor is how accurately the intake and exhaust valves can follow their associated rocker arms and how accurately push tubes, lifters, and rockers can follow lobes on a camshaft. As engine rpm increases, a point is eventually reached at which these valve train components start to float. That is, they are no longer able to follow their inputs or adjoining components. If an intake or exhaust valve starts to float, the possibility of interference with the corresponding cylinder piston becomes real, and the occurrence of such interference or collision between a valve and piston can damage or even destroy an engine.
An engine can reach such a limiting rpm under two conditions. First, if the engine is afforded the right combination of fuel, air, ignition spark, load, etc., it will accelerate to such limit on its own. Secondly, the engine can be motored to this limit when an outside force increases the rpm to one that is higher than the engine would normally attain at a specific air and fueling rate; in other words, the engine is being pushed faster than it wants to go.
A prime example of how an engine can be motored is when the vehicle is going down a hill or mountain while in gear with the operator's foot off the accelerator pedal. If gravity is pulling the vehicle down the grade harder than the engine and associated vehicle components are trying to slow the vehicle, the vehicle speed and engine rpm will be increased, and under certain conditions the degree of motoring will be sufficient to drive the engine to a speed at which engine damage can occur.
Another common instance where an engine can be motored beyond a safe rpm may occur during downshifting of a manual transmission. If the transmission is placed in too low a gear for the vehicle speed, the engine will be motored to too high a speed when the clutch is re-engaged at the conclusion of the shift.
In some vehicles the engine is equipped with a governor on the fuel, air, or ignition system to limit engine speed for fuel economy or other purposes. Such governors are effective in limiting engine rpm under non-motoring conditions, but are ineffective in limiting engine rpm when the engine is being motored.